4.8.4 Subatomic particles

Classification

Subatomic particles are classified according to whether
they do or do not respond to the strong nuclear force. Those that do are named
‘hadrons’, of which the protons and neutron are particular
examples, while those that do not respond to the strong force are called
‘leptons’, and the electron and neutrino are examples.

The leptons appear to be pointlike even when probed to
the highest resolution currently available ( 10−18 m). The hadrons are known to be extended
objects ( 10−15 m diameter) which are built from
pointlike particles known as ‘quarks’. Five varieties of quark have
been identified which are distinguished by their electrical charges and further
intrinsic properties named ‘strangeness’, ‘charm’,
‘bottom’ (or ‘beauty’) which are additive quantum
numbers. Thus a hadron containing two charmed quarks will have net 2 units of
charm and is referred to as a charmed hadron; the more strange quarks there are
contained in a hadron, the more will be its net strangeness. The total
electrical charge of a hadron is the sum of the electrical charges of its
constituent quarks.

The quarks respond to the strong nuclear force but
otherwise behave very much like leptons. This has led to conjectures that
leptons and quarks may be fundamental and related to one another, though the
precise nature of this relationship is still unclear. Recent evidence for a
sixth variety of quark (variously called the ‘top’ quark or
sometimes ‘truth’) completes a parallelism with the six
leptons.

For each variety of particle listed nature also has an
antiparticle with the same mass and spin but opposite charge, strangeness and
other additive quantum numbers. Their symbols are the same as those for their
particle equivalents but for a bar above them, e.g. p is a proton and
is the antiproton. The anti-electron is denoted
−, or most commonly
e+, and named the ‘positron’.

The data on antiparticles are much poorer than for
particles. General theorems require that antiparticles have the same mass and
lifetimes as their particle equivalents: the poor data are all consistent with
this and are not listed here.

In addition, there are particles classified by the
family name ‘gauge bosons’. These are carriers of the fundamental
forces and have spin. The photon is the carrier of the electromagnetic force;
the W and Z bosons are the carriers of the weak forces.

Fundamental particles with spin

Leptons

Name and
symbol

Charge

Mass/MeV

Mean life/s

Principal decay modes

Electron

e

−e

0.510 999 06 ± 0.000 000
15

Stable

Stable

(> 1.9 × 1023
years)

Electron-

νe

0

< 7.3 ×
10−6

Stable

Stable

neutrino

Muon

μ

−e

105.658 389 ± 0.000 034

2.197 03 ×
10−6

ev

± 0.000 04

Muon-

vμ

0

< 0.27

Stable

Stable

neutrino

Tau

τ

−e

1777.1

+ 0.4

− 0.5

2.956 × 10 −13
± 0.031

Hadron + neutralsπ−π0ν,
μνν, eνν

Tau-

ντ

0

< 31

Stable

Stable

neutrino

 The observed mean life of a
particle in flight is longer than its mean life at rest by a factor

(1−β2) −
1/2 = 1 +

Kinetic enery

Rest energy

where β is the ratio of its velocity in the
observer's frame to the velocity of light.

Quarks

Name
and symbol

Charge

Z-component (IZ) of isospin

Baryon number

Other non-zeroquantum numbers

Mass/GeV

Down

d

−

1

e

3

−

1

2

1

3

0

0.35

Up

u

2

e

3

+

1

2

1

3

0

0.35

Strange

s

−

1

e

3

0

1

3

Strangeness − 1

0.5

Charm

c

2

e

3

0

1

3

Charm + 1

1.3−1.7

Bottom

b

−

1

e

3

0

1

3

Bottom + 1

4.7−5.3

Top

t

2

e

3

0

1

3

Top + 1

175

No individual quarks have been isolated so the
concept of mass is not well defined. The listing is merely a qualitative guide
extracted from the masses of the lightest hadrons built from the respective
quarks. For more details see F. E. Close (1979) An Introduction to Quarks
and Partons, Academic Press.

Stable and metastable hadrons

These data refer only to particles immune to decay via
the strong interaction; they are derived by the Particle Data Group (Phys.
Rev., D50, July 1994) which contains a considerable expansion of the
data of this table together with data on unstable mesons and baryons. At
present these data are updated biennially by the Particle Data Group.

The quark content is listed by symbols, e.g. the proton
built of two up quarks and one down quark is denoted uud. Several
metastable heavy particles have been found built from a charmed quark
(c) and charmed antiquark (
) or from a
bottom quark (b) and a bottom antiquark (
). The
resulting bound states of these quarks appear to be effectively
non-relativistic systems with spin 0 or 1 and orbital angular momentum similar
to positronium. By analogy they are known as ‘charmonium’ and
‘bottomonium’ respectively. The spectrum is listed under Heavy
quark spectroscopy, below. The charmonium and bottomonium states all have zero
baryon number, strangeness, charm and bottom quantum numbers.

The charm (C), strangeness (S) and baryon number (B) of
the hadrons which appear in the subsequent tables are as follows:

Hadron type

Mesons (B = 0)

S

C

Baryons (B = 1)

S

C

Non-strange . . . . . .

π, η

0

0

p, n

0

0

Strange
. . . . . . .

K+

+1

0

Λ, Σ

−1

0

K−

−1

0

Ξ

−2

0

K

0

,

s

K

0

L

~ 50%

~ 50%

+1

0

Ω−

−3

0

−1

0

Charmed . . . . . . .

D+, D0

0

1

Λ

+

c

0

+1

D− ,
0

0

−1

D

+

s

+1

+1

D

−

s

−1

−1

Mesons consist of a quark and an antiquark; baryons (B = 1) consist of
three quarks (each with B =
). The superscripts denote charges in units of the proton
charge.

Nonstrange hadrons

Name
and symbol

Quark content

Spin

Mass/MeV

Mean life/s

Principal modes of decay

Pion π+, (π−)

u(dū)

0

139.5699 ± 0.00035

2.6030 × 10−8 ± 0.0024

μ±ν

π0 . . . . . .

uū
and d

0

134.9764 ± 0.0006

0.84 × 10−16 ±
0.06

γγ

Eta
η0 . . . . .

uū, d and s

0

547.45 ± 0.19

7.93 × 10−19 ±
1.1

γγ,

π0π0π0,

π +π −π0

Proton p
. . .

uud

1

2

938.272 31 ± 0.000 28

Stable (> 1.6 × 1025 years)

Stable

Neutron n
. . .

ddu

1

2

939.5653† ± 0.00028

887.0 ± 2.0

pe−

† The difference (mp −
mn) between the proton and neutron masses is known very
accurately to be − 1.293 318 ± 0.000 009 MeV.

Strange hadrons

Name
and symbol

Quark content

Spin

Mass/MeV

Mean life/s

Principal modes of decay

K-mesons

K + (K
−) . . .

u, (sū)

0

493.677 ± 0.016

1.2371× 10 −8 ± 0.0029

μ±ν,
π±π0

K

0

S

K

0

L

~

50% s

~

50% d

0

497.671 ± 0.031

0.8922 × 10 −10 ± 0.0020 5.17
× 10 −8 ± 0.04

π+π−,
π0π0π0π0π0, π+π−π0 π±ev,
π±μν

Hyperons

Λ . . . .

uds

1

2

1115.68 ± 0.01

2.632 × 10 −10 ± 0.020

pπ−, nπ0

Σ+ . . . . .

uus

1

2

1189.36 ± 0.06

0.800 × 10 −10 ± 0.004

pπ0, nπ+

Σ− . . . . .

dds

1

2

1197.34 ± 0.05

1.482 × 10 −10 ± 0.011

nπ−

Σ0 . . . . .

uds

1

2

1192.46 ± 0.08

5.8 × 10 −20 ± 1.3

Λγ

Ξ− . . . . .

dss

1

2

1321.32 ± 0.13

1.641 × 10 −10 ± 0.016

Λπ−

Ξ0 . . . . .

uss

1

2

1314.9 ± 0.6

2.90 × 10 −10 ± 0.10

Λπ0

Ω− . . . . .

sss

3

2

1672.45 ± 0.32

0.819 × 10 −10 ± 0.027

ΛK−,
Ξ0π−,
Ξ−π0

The difference (MKL − MKS)
between the K0L and K0S masses is known very accurately to be MKL − MKS = (3.510 ±
0.018) × 10 −12 MeV

The massive charmed and bottom quarks form
non-relativistic bound states with their corresponding antiquarks. The
resulting spectroscopy is similar to that of positronium and is known as
‘charmonium’ (charmed quark and charmed antiquark) or
‘bottomonium’. The low-lying energy levels are metastable. These
energy levels yield important information on the nature of the interquark
forces. The quark and antiquark couple their spins to a total of spin 0 or 1
and are in a state of relative orbital angular momentum L. The spectrum
is listed in standard 2S +
1LJ notation.